dt: bindings: Add bindings for SDHCI Synopsys DWC MSHC

Synopsys SDHCI compatible DesignWare Cores Mobile Storage Host
Controller can support eMMC/SD/SDIO. Add the bindings.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>